Generally, golf balls have been classified as two-piece balls, wound (or three-piece) balls, and balls with multi-layer cores and/or multi-layer covers wound balls or solid balls. The prior art comprises a variety of golf balls that have been designed to provide particular playing characteristics. These characteristics are generally the initial velocity and spin of the golf ball, which can be optimized for various types of players. For instance, certain players prefer a ball that has a high spin rate in order to control and stop the golf ball. Other players prefer a ball that has a low spin rave and high resiliency to maximize distance. Generally, a golf ball having a hard core and a soft cover will have a high spin rate. Conversely, a golf ball having a hard cover and a soft core will have a low spin rate. Golf balls having a hard core and a hard cover generally have very high resiliency for distance, but are hard feeling and difficult to control around the greens.
Two-piece golf balls are generally most popular with the average recreational golfer because these balls provide durability while allowing greater distance. These balls contain a solid core, typically made of crosslinked polybutadiene, and a cover, typically made of a tough, cut-proof ionomer resin, such as SURYLN.RTM. (DuPont). The combination of the materials used to construct the core and cover provides a ball that is virtually indestructible by golfers. Further, such combination imparts a high initial velocity to the ball, which results in improved distance during play. The rigidity of the materials used also provides two-piece golf balls with a harder "feel" when struck with a club and results in relatively a low spin rate which maximizes travel distance. However, greater distance is achieved at the expense of decreasing control, particularly on shorter approach shots.
The simple structure of the two-piece golf balls renders these balls relatively easy to manufacture by using well-known, conventional methods. Generally, a polybutadiene rubber composition is mixed and formed into preparatory materials ("preps"). These preps are then inserted into a compression mold to form spherical cores. Thereafter, a cover is injection molded or compression molded over the cores to form the finished golf balls.
Three-piece golf balls, or wound balls, are the preferred balls for more advanced players and are generally thought of as performance golf balls. Wound balls are more resilient and are associated with greater spin characteristics and softer feel when struck by a golf club. Wound balls are generally constructed fiom a liquid or solid centers surrounded by tensioned elastomeric material. The wound core is then covered with a durable cover material, such as SURLYN.RTM. or a similar material, or a softer cover such as Balata or polyurethane. Wound balls have a generally softer "feel" and provide more spin during travel, which enables a skilled golter to have more control over the ball's flight and final position. However, wound, higher-spinininig balls typically provide a shorter distance compared to two-piece balls. Moreover, as a result of their more complex structure, wound balls generally require more time and expense to manufacture than two-piece balls.
In an effort to maximize the benefits of two-piece balls and wound balls, golf ball manufacturers have been modifying the properties of conventional non-wound balls by altering the typical single layer core and single layer cover construction to provide a multi-layer ball. These multi-layer cores provide new and improved characteristics to the resulting golf balls. However, they introduce new demands on the manufacturing process. In making solid balls manufacturers have molded layers around a solid center by placing a pre-formed center between two blocks of core material in a spherical compression mold. This process, however, provides minimal control over the ultimate placement of the center within the golf ball and often results in large variations in the alignment of the center of the core with respect to the center of the golf ball.
There are prior art patents directed to improved manufacturing processes for making golf balls. For example, U.S. Pat. No. 5,006,297 discloses a method in which partially cured polyurethane half-shells are formed around a center in a fixed-pin mold. The half-shells and the center are intermediately cured and compression molded thereafter to form a finished golf ball. This method, however, is Generally inapplicable to forming multi-layer cores of different formulations. U.S. Pat. No. 5,314,187 discloses another method for golf ball manufacture in which an inner cover layer is molded around a core, either by injection molding around the core or by injection molding twvo half shells, and compression molding the cover layer around the core. U.S. Pat. No. 5,586,950 discloses a similar method. However, these patents are directed to inner cover layers constructed of thermoplastic materials.
The prior art also includes a method for the manufacture of double cover golf balls. This is generally accomplished by injection molding a first and then a second cover layer around a core. This system, however, requires complex injection molds, usually with retractable pins within the mold to properly position the core. Moreover, this system is better suited for thermoplastic materials.
Multiple outer layers are conventionally formed around a golf ball center by a variety of methods, including compression molding or injection molding a spherical shell layer around the center. Each of these manufacturing methods suffers from several disadvantages. Injection molding of the shells around the golf ball center is expensive, due to the higher capital costs associated with the more complex manufacturing process. In addition, injection molding can introduce defects into the core due to the penetration by the pins. Moreover, injection molding does not generally work well with thermoset materials. An alternative method was developed wherein hemispherical shells are formed first and are thereafter compression molded around a golf ball center to avoid pin defects. This method generally works well with thermoplastic materials. However, this method is not generally contemplated with thermoset materials, which lack sufficient rigidity to hold a formed shape before being cured and cannot be reformed after being cured. Thus, the multi-layer cores formed today generally have poor concentricity between the center and outer layers, which adversely affects playing characteristics. In addition, the prior art fails to provide a processes which can improve the properties of multi-layer cores using a variety of core formulations.
Thus, there remains a need for a method directed to improving the manufacturing processes used for making multi-layer cores that is adaptable for a variety of golf ball core formulations and produces golf ball cores with improved centering characteristics.